1. Technical Field of the Invention
The present invention generally relates to a programmed shutter and to an apparatus for preventing fine particles of foreign matter, e.g., dust, which is located on a lens, from being photographed; and more specifically to such an apparatus which prevents photographing of foreign matter, such as dust, on a lens in a lens shutter tyoe of camera having a zoom lens system.
2. Description of Related Art
In conventional cameras, even when foreign material is located on a camera lens, there is no possibility that the foreign matter will be photographed in the picture, provided that such matter comprises fine material. This is because the area which is occupied by the fine foreign matter is extremely small with respect to the rays of light which define the picture (i.e., the image), such that any image of such foreign matter will be dimmed due to the diffraction of light.
However, in a zoom lens system in which the position of the diaphragm of the camera varies, such as has been developed by the assignee of the present application, the exit pupil of the camera closely approaches the film plane at one of the focal length extremities of the camera. Accordingly, at a small value of the disphragm, in which the diameter of the exit pupil is below a predetermined value, i.e., when the lens system closely approaches the film plane, foreign matter which is located on the rearmost lens adjacent the center of the lens will be photographed in the picture. A zoom lens camera such as that referred to above is the subject of PCT patent application Ser. Nos. PCT/JP87/00292 and PCT/JP87/00293, both of which were filed on May 12, 1987, and both of which are expressly incorporated by reference herein.
FIGS. 7A 7B, and 7C all show how foreign matter on the rear surface of a lens is photographed when a picture is taken.
Specifically, FIGS. 7A and 7B illustrate a lens system L which closely approaches the film plane F, when the lens system is at its minimum focal length. In FIG. 7A, foreign matter cannot be photographed when taking a picture, and, in FIG. 7B, foreign matter will be photographed as a black point in the picture when the photograph is taken. Assuming that foreign matter Q, having a diameter of d .phi., is applied to the center portion of the rearmost lens of lens system L, when exit pupil P of lens system L is small and when the distance between the rearmost lens and film plane F is also small, the quantity of light on the center of film plane F will be decreased by the presence of the foreign matter thereon. Such a decreased quantity of light will effect a phenomenon in which the foreign matter is photographed. When the diameter of the foreign matter is 0.3 mm .phi., which is the most likely diameter of material which will be located on lens system L, the minimum exit pupil P which will not recognize such foreign matter is, e.g., F13.5 (see FIG. 7A). When the exit pupil P is smaller than F13.5, e.g., when the exit pupil P is F22, foreign matter can be clearly recognized and will be photographed, as shown in FIG. 7B.
To the contrary, when lens system L moves forwardly towards a maximum focal length, as shown in FIG. 7C, no problem of decreased light resulting from the presence of foreign matter Q will occur, even if exit pupil P is F22, which is identical to the diaphragm aperture at the minimum focal length of the camera. Specifically, when the distance between the lens system and film plane F is above a predetermined value, no foreign matter can be photographed.
Recently, with zoom lens being used having increased magnifications, and with minimization of the size of the lens system, it has occurred that the rearmost position of the lens system and of the exit pupil more closely approach the film plane. This results in an increased possibility that foreign matter will be photographed in pictures being taken with such cameras.
In another known camera, in which a focus switching lens moves in and out of the optical path of the lens system in order to selectively provide maximum and minimal focal lengths for the lens system, such a problem (photographing of dirt and other particles) also occurs.